Die grondseil en slaapsak is droog.

Because Afrikaans uses is for the present tense of to be with both singular and plural subjects.

So:

• Die slaapsak is droog. = The sleeping bag is dry.
• Die grondseil en slaapsak is droog. = The groundsheet and sleeping bag are dry.

Unlike English, Afrikaans does not switch between is and are in the present tense.

Both are possible.

• Die grondseil en slaapsak is droog.
• Die grondseil en die slaapsak is droog.

With one die, the two nouns are grouped together a bit more naturally, especially when they belong together in context. English does this too sometimes: the groundsheet and sleeping bag.

Using die before both nouns is also correct and can sound slightly more explicit.

Here droog is an adjective used after the verb is, so it is a predicate adjective.

Structure:

• Die grondseil en slaapsak = subject
• is = verb
• droog = adjective describing the subject

So the sentence pattern is simply:

subject + is + adjective

Very similar to English: The groundsheet and sleeping bag are dry.

Because droog comes after the verb is.

In Afrikaans, adjectives often change form when they come before a noun, but usually stay in their basic form when they come after a linking verb like is.

Compare:

• Die slaapsak is droog. = The sleeping bag is dry.
• die droë slaapsak = the dry sleeping bag

So in your sentence, droog is correct because it is not directly in front of a noun.

Yes. Afrikaans very often joins related nouns into one compound word.

• grond = ground
• seil = sheet / sail / canvas, depending on context
• grondseil = groundsheet

Likewise:

• slaap = sleep
• sak = bag
• slaapsak = sleeping bag

This is very common in Afrikaans, much like German and Dutch.

Sure.

• grondseil literally breaks down as ground + sheet/canvas
• slaapsak literally breaks down as sleep + bag

Even if the full meaning is already known, learners often find it helpful that Afrikaans compounds are very transparent.

Yes. This is standard Afrikaans word order for a simple statement:

Die grondseil en slaapsak + is + droog

That is:

subject + verb + complement

Nothing unusual is happening in this sentence.

Yes. Afrikaans uses die as the definite article for both singular and plural.

Examples:

• die slaapsak = the sleeping bag
• die slaapsakke = the sleeping bags

So die does not tell you by itself whether something is singular or plural. You understand that from the noun, context, or the whole phrase.

You know it is plural from the subject:

Die grondseil en slaapsak = two things joined by en (and)

That makes the subject plural in meaning, even though the verb stays is.

So Afrikaans relies more on the subject and context here, not on a special plural form of the verb.

Yes, absolutely. That version is fully correct.

The difference is mostly stylistic:

• Die grondseil en slaapsak is droog. = a little more compact
• Die grondseil en die slaapsak is droog. = a little more explicit

Both mean the same thing.

A helpful rough guide for English speakers is:

• grondseil ≈ gront-sail
• slaapsak ≈ slahp-suck
• droog ≈ something like drohkh or drohch, with a final guttural sound not found in standard English

A few notes:

• aa in slaapsak is a long vowel.
• ei in grondseil sounds somewhat like the vowel in English day or say, though not exactly the same.
• The final g in droog is the usual Afrikaans guttural g/ch sound.

It can describe both together perfectly naturally. Afrikaans does not need to repeat it.

So:

• Die grondseil en slaapsak is droog.

means both the groundsheet and the sleeping bag are dry.

You would only repeat it if you wanted extra emphasis or a different structure, for example in a longer sentence.